Hitherto, bituminous coal (high-grade coal) has been often used as a coal fuel used for combustion in coal-fired boilers. However, in recent years, attempts have been increasingly made to combust low-grade coal (for example, subbituminous coal, brown coal, or the like) in the coal-fired boilers from the viewpoints of resource depletion issues, the expansion of resource options, and reductions in fuel costs.
However, the used of the low-grade coal for combustion in the coal-fired boilers has problems below. First, the ash of the low-grade coal contains large amounts of basic components. In combustion in the coal-fired boilers, combustion ash adheres to furnace walls, heating tubes, and the like in the boilers. The adhesion of combustion ash is likely to be caused when the content of each basic component contained in ash is high. As a result, the heat absorption efficiency may possibly be reduced. In addition, the low-grade coal contains a large amount of moisture. Therefore, the drying load of a crusher (mill) may possibly be increased. Furthermore, the low-grade coal has a lower heating value as compared to bituminous coal. From the above, the low-grade coal has lower combustion efficiency as compared to bituminous coal.
Therefore, a method for blending bituminous coal and low-grade coal is known as a technique attempted to solve the above problems. For example, Patent Literature 1 discloses a method for suppressing ash adhesion in a coal-fired boiler in such a way that a mixture prepared by mixing the combustion ash of bituminous coal containing small amounts of basic components with coal (Adaro coal (low-grade coal)) containing ash containing large amounts of basic components is used as a coal-blended fuel. As a result, it is cited that the basic components are diluted and therefore ash adhesion is reduced.